Printing images onto a print medium such as paper for consumer and industrial use is dominated generally by laser technology and ink jet technology. Ink jet technology has become more common as ink jet printing resolution and quality have increased. Ink jet printers typically use either thermal ink jet technology or piezoelectric technology. Even though they are more expensive to manufacture than thermal ink jets, piezoelectric ink jets are generally favored because they can use a wider variety of inks.
Piezoelectric ink jet printheads include an array of piezoelectric transducers attached to a flexible membrane. Other printhead structures can include one or more laser-patterned dielectric standoff layers and a flexible printed circuit (“flex circuit”) or printed circuit board (“PCB”) electrically coupled with each transducer. A printhead can further include a body plate, an inlet/outlet plate, and an aperture plate, each of which can be manufactured from stainless steel. The aperture plate includes an array of nozzles (i.e., one or more openings, apertures, or jets) through which ink is dispensed during printing.
The transducers of a printhead generally reside adjacent to a pressure chamber. A set of signals generally cause the transducer to act against a diaphragm. One signal causes the transducer to move the diaphragm in a direction away from the aperture, filling the pressure chamber with ink. A second signal, typically of opposite polarity of the first, causes the diaphragm to move the other direction, pushing ink out of the pressure chamber through the aperture. In other words, during use of a piezoelectric printhead, a voltage is applied to a piezoelectric transducer, typically through electrical connection with a flex circuit electrode electrically coupled to a voltage source, which causes the piezoelectric transducer to bend or deflect, resulting in a flexing of the diaphragm. Diaphragm flexing by the piezoelectric transducer increases pressure within an ink chamber and expels a quantity of ink from the chamber through a particular nozzle in the aperture plate. As the diaphragm returns to its relaxed (i.e., unflexed) position, it reduces pressure within the chamber and draws ink into the chamber from a main ink reservoir through an opening to replace the expelled ink.
FIGS. 1A and 1B illustrate one example of a single inkjet ejector 110 that is suitable for use in an inkjet array of a print head. The inkjet ejector 110 has a body 122 that is coupled to an ink manifold 112 through which ink is delivered to multiple inkjet bodies. The body also includes an ink drop-forming orifice or nozzle 114 through which ink is ejected. In general, the inkjet print head includes an array of closely spaced inkjet ejectors 110 that eject drops of ink onto an image receiving member (not shown), such as a sheet of paper or an intermediate member.
Ink flows from the manifold to nozzle in a continuous path. Ink leaves the manifold 112 and travels through a port 116, an inlet 118, and a pressure chamber opening 120 into the body 122, which is sometimes called an ink pressure chamber. Ink pressure chamber 122 is bounded on one side by a flexible diaphragm 130. A piezoelectric transducer is secured to diaphragm 130 by any suitable technique and overlays ink pressure chamber 122. Metal film electrode layers 134, to which an electronic transducer driver 136 can be electrically connected, can be positioned on either side of a piezoelectric element 132. The metal film layers can be patterned, in a manner such that the piezoelectric transducers can be addressed individually or as groups with various numbers of elements in each group.
Ejection of an ink droplet is commenced with a firing signal. The firing signal is applied across metal electrode layers 134 to excite the piezoelectric element 132, which causes the transducer to bend. Because the transducer is rigidly secured to the diaphragm 130, the diaphragm 130 deforms to urge ink from the ink pressure chamber 122 through the outlet port 124, outlet channel 128, and nozzle 114. The expelled ink forms a drop of ink that lands onto an image receiving member. Refill of ink pressure chamber 122 following the ejection of an ink drop is augmented by reverse bending of piezoelectric element 132 and the concomitant movement of diaphragm 130 that draws ink from manifold 112 into pressure chamber 122.
Generally, one transducer exists for each aperture and pressure chamber, and the array of transducers aligns to the arrays of pressure chambers. The desire for high resolution print images has driven the density of the array of apertures increasingly higher. The array of transducers has to match this higher density. For example, the number of apertures corresponds to the number of body cavities, which in turn correspond to the number of transducers. These higher densities lead to extremely tight tolerances during manufacture of a print head.
Piezoelectric ink jet printheads may include an array of piezoelectric elements to form the piezoelectric transducers. One process to form the array can include detachably bonding a blanket piezoelectric layer to a transfer carrier with an adhesive, and dicing the blanket piezoelectric layer to form a plurality of individual piezoelectric elements. A plurality of dicing saw passes can be used to remove all the piezoelectric material between adjacent piezoelectric elements to provide the correct spacing between each piezoelectric element.
A top electrode formed on each transducer provides electric coupling of the piezoelectric transducers to corresponding voltage sources. In current methods for fabrication, the top electrode may be formed in a plating process in which a blanket coating of electrode metal is deposited over an entire slab of piezoelectric material, followed by dicing of the slab to form individual actuators (i.e., individual actuators are defined by dicing rows and columns out of the electrode coated piezo material). One issue with such a method of blanket coating is that the dicing step stresses the adhesion of the metal electrode due to physical contact with the saw and exposure to high pressure cooling water. Another issue with such a method of blanket coating is related to the handling of thin piezoelectric material. To achieve higher density transducer arrays, the piezoelectric material must be smaller and thinner. Blanket coating two sides of very thin piezoelectric material (e.g., 0.010 mm-0.030 mm) can be extremely challenging. Other challenges for a blanket coating process are presented when piezoelectric transducers are fabricated by using a thick-film process: a process by which there is no slab to start with and the piezoelectric material is stenciled or printed as individual tiles onto the substrate. With a thick-film process, blanket coating of all of the singulated tiles would result in unintended electrical paths (e.g., actuator to actuator, actuator to ground, etc.)
What is needed, therefore, is a method for assembling a printhead that minimizes or eliminates use of blanket coating of piezoelectric stacks in order to minimize or eliminate the above issues.